In my co-pending application entitled "PACEMAKER PROGRAMMER-BASED AUTOMATIC RETROGRADE CONDUCTION MEASUREMENT", Ser. No. 08/226,175 filed on even date herewith, which application is hereby incorporated by reference, there is described a technique by which retrograde conduction in a pacemaker patient may be measured automatically by using a physician-operated pacemaker programmer. The measurement of retrograde conduction intervals allows the setting of an appropriate postventricular atrial refractory period (PVARP) to avoid retrograde conduction which might give rise to the onset of pacemaker mediated tachycardia (PMT).
The existence of retrograde conduction via the natural pathway, and the antegrade conduction via the implanted dual-chamber pacemaker, provide a reentry circuit that mimics the natural situation in which an accessory pathway allows a circus movement tachycardia. This reentry tachycardia, PMT, may occur when a P-wave displaced from its natural position before the QRS complex is tracked in the ventricle. The P-wave can be displaced by a ventricular premature contraction or ventricular stimulus with retrograde conduction. If the displaced P-wave falls within the atrial refractory period, it will not be tracked and no further event will occur. On the other hand, if the displaced P-wave falls outside the atrial refractory period, it will begin an AV interval and be tracked in the ventricle. If retrograde conduction exists, another P-wave will occur following the ventricular stimulus, and the reentry loop will be sustained.
If retrograde conduction exists, PMT can be prevented by programming the PVARP longer than the retrograde conduction interval. The retrograde conduction time begins with a ventricular event. The retrograde conduction time ends when a retrograde P-wave occurs (assuming that the retrograde conduction results in an atrial event). The PVARP interval similarly begins with the ventricular event. Thus, PMT can be prevented by making the PVARP interval longer than the retrograde conduction interval because the atrial event will not be tracked (although it can be sensed) if it occurs during the PVARP. On the other hand, higher tracking rates can be programmed if the PVARP is shortened. Therefore, to provide an optimal trade-off between these two factors, the retrograde conduction interval should be measured if it exists.
My aforesaid co-pending application discloses how a patient's retrograde conduction interval may be measured automatically by using a pacemaker programmer to control cycling of the pacemaker and to display information telemetered from the pacemaker. The pacemaker first generates a series of ventricular pacing pulses, and it measures the interval between each V-pace and the subsequent atrial beat (A-sense). If the measured intervals are consistent, it is possible that there is retrograde conduction. To confirm that the A-senses were not due to noise, atrial premature beats, etc., subsequent ventricular pacing pulses are generated in a manner that eliminates the possibility of atrial beats being due to retrograde conduction: a series of simultaneous atrial and ventricular pacing pulses are generated. If A-senses consistent with the previously measured intervals are not sensed, retrograde conduction is confirmed, i.e., the previously detected A-senses must have been due to retrograde conduction. The PVARP of the pacemaker can then be set to exceed the longest measured V-pace to A-sense (VP-to-AS) interval so that the effect of retrograde conduction is minimized. Any retrogradely conducted signal falls in the PVARP where it is ignored by the pacemaker. Since PMT arises from the sensing, early in the pacemaker cycle, of a retrogradely conducted signal, PMT can be avoided by extended the PVARP so that a retrogradely conducted signal does not trigger the usual AV delay and generation of a ventricular pacing pulse.
The technique disclosed in my aforesaid co-pending application is preferably controlled by a pacemaker programmer. It is an aid to the physician to allow him to set an appropriate PVARP interval to minimize the occurrence of PMT. But if PMT does occur, for example, if the retrograde conduction time changes, it would be highly advantageous not only to confirm its presence in a reliable way, but to automatically terminate it.